1. Field of the Invention
The present invention relates to a method for growing a ternary or quaternary group II-VI compound semiconductor layer having a precisely controlled composition by vapor phase epitaxy and a product using the same. More particularly, the invention relates to a method for growing an epitaxial layer of mercury cadmium telluride (HgCdTe) on a sapphire substrate, and to a sensitive device incorporating the HgCdTe layer on the sapphire substrate. 2. Description of the Related Art
Well known conventional methods for growing HgCdTe layers are first explained using schematic FIGS. 1(a) and 1(b). As shown in FIG. 1(a), a cadmium telluride (CdTe) substrate 2 on which a HgCdTe layer is to be grown and a source 1 comprising a melt of mercury (Hg), cadmium (Cd) and tellurium (Te) elements are disposed at opposite respective ends in a tubular enclosure 3. FIG. 1(b) is an illustration of a temperature distribution curve 4 showing the temperature at corresponding positions of FIG. 1(a) after the tubular enclosure 3 has been put into a furnace(not shown). Source 1 is first prepared by providing the above three elements, and the weight of each element in the source is determined so as to satisfy the stoichiometric composition of the Hg.sub.1-x Cd.sub.x Te layer. The three elements are mixed, melted and finally solidified to form the source 1. In a stoichiometric composition the total amount of mercury and cadmium atoms is substantially equal to the total amount of tellurium atoms, and each of the mercury and cadmium atoms is combined with a tellurium atom so as to form both HgTe and CdTe.
As shown in FIG. 1(b), the temperature of source 1 is about 600.degree. C. and this temperature is a little higher than the temperature of other regions in enclosure 3 by about 10.degree. C. Under these conditions, the vapor pressure inside tubular enclosure 3 is greater than about 8 atm, and in order to avoid the danger of enclosure breakage, the dimensions of tubular enclosure 3 and of the substrate 2 cannot be sufficiently large.
J. G. Fleming et al., in their paper "Control of the surface composition of isothermal vapor phase epitaxial mercury cadmium telluride", J Vac Sci Technol A5(6), November/December 1987, disclose a method for controlling the surface composition of a mercury cadmium telluride system during isothermal vapor phase epitaxy of mercury cadmium telluride. The method employs a source in three-phase (solid-liquid-vapor) equilibrium for the desired composition and by using this method it is possible to avoid the high vapor pressure conditions of the prior art. The Fleming et al. method and composition profiles of the layers grown thereby are outlined in FIGS. 2 and 3.
FIG. 2 presents a cross-sectional view of the structure used for the tests. The structure comprises a quartz ampoule 5 and a quartz cover plug 10 forming an enclosure. A tellurium-rich source 6 is disposed at the bottom of the ampoule 5, and a CdTe substrate 8 is held in a fixed position above the source 6 with the spacing therebetween determined by quartz spacers 7 as shown.
The tellurium-rich source 6 plays an important role in the growth of the mercury-cadmium-tellurium layer. The source 6 comprises a solid mass of stoichiometric mercury cadmium telluride having the composition Hg.sub.1-x Cd.sub.x Te and a tellurium-rich (hereinafter abbreviated as Te-rich) liquid consisting of a non-stoichiometric mercury cadmium telluride mixture having a composition of (Hg.sub.1-z Cd.sub.z).sub.1-y Te.sub.y wherein the Te-rich liquid is defined by a value of y which is larger than 0.5. The ratio of solid to liquid is about 9/1. At the growth temperature, the source is maintained in three phase solid-liquid-vapor equilibrium.
FIG. 3 shows the composition profile of the thusly grown layers. The distance from the growth surface is plotted along the abscissa and the x'-value of epitaxial Hg.sub.1-x' Cd.sub.x' Te layers grown using different source compositions is plotted along the ordinate. The x-values shown in FIG. 3 for curves 11 through 16 are the x-values of the solid Hg.sub.1-x Cd.sub.x Te composition used as the three-phase solid-liquid vapor equilibrium source.
As can be seen from FIG. 3, when the parameter x of the solid in the source is equal to 0.2 the x' values of the grown layers are always larger than 0.2. Even in the extreme case where x=0, which means that the source comprises solid mercury telluride (HgTe) and Te-rich liquid mercury telluride, the grown layer comprises a cadmium component (x'.gtoreq.0.12) and has a composition represented by the formula Hg.sub.0.88 Cd.sub.0.12 Te. Thus, when the foregoing method is used, it is not possible to grow mercury cadmium telluride epitaxial layers wherein the mole fraction of CdTe is equal to or less than 0.11. That is to say, it is not possible to grow Hg.sub.1-x Cd.sub.x Te layers having an x-value that is 0.11 or less.
Fleming's method for growing HgCdTe epitaxial layers as outlined above utilizes a source comprising a mixture of a solid stoichiometric HgCdTe compound having a predetermined composition and a Te-rich liquid HgCdTe melt wherein the components are in three-phase equilibrium at the growth temperature. The preparation of source materials which will grow an epitaxial Hg.sub.1-x Cd.sub.x Te layer on a CdTe substrate, wherein the layer will have specified mole fraction x of CdTe in the ternary Hg.sub.1-x Cd.sub.x Te compound, necessitates complicated process steps. When the composition of the grown layer is to be changed, each of the source materials must be prepared again.